As shown in FIG. 1, the charging system of a typical motor vehicle consists of an alternator, voltage regulator, and battery. Internal to the alternator are the field coil (rotor), stator, rectifier bridge and the diode trio.
When current flows through the field coil, a magnetic field is produced. When the alternator shaft is rotated by the engine, a voltage is induced in the stator. The rectifier bridge converts AC voltage from the stator to a DC voltage needed for the charging of the vehicle's battery. The diode trio functions the same as the rectifier bridge but its output is used to supply current to the field coil. The voltage regulator limits the alternator output voltage to a preset level by controlling the field coil current.
On initial engine start-up, residual magnetism in the field coil generates a voltage in the stator which is rectified by the diode trio. This voltage is applied to the field coil by the voltage regulator and current flows through the field coil increasing the induced stator voltage and subsequently the voltage available for the field coil current. Since the rectifier bridge and diode trio anodes are connected commonly to the stator, the output voltage from the alternator and the voltage available to the field coil are approximately the same.
It is known in the past that at low engine speeds, the available output current from the alternator decreases due to the decrease in induced stator voltage. The voltage regulator attempts to maintain a constant voltage at the alternator output by increasing field coil current but is limited by the available voltage and magnetic saturation. As the stator voltage decreases, the resistance of the field coil will limit the field coil current proportionally to the voltage applied to the field coil.
The present invention provides a field voltage boost circuit that can be used between the diode trio and the field coil to increase the maximum available field current by increasing the voltage applied across the field coil. Increased output power from the alternator is achieved by this increased field current.